This invention relates generally to high voltage solid state switches and, more particularly, to switches used as pulse modulators for cathode pulsed tubes used in amplifying radio frequency signals.
As is known in the art, it is sometimes desirable, as in radar transmitters, to produce amplified pulses of radio frequency energy. One such technique includes feeding a pulse modulating signal to a modulator circuit which electrically couples, or decouples, a power supply to or from the cathode-anode of a crossed field tube selectively in accordance with the modulating signal. Such crossed field tube may be a magnetron, a klystron or a crossed field amplifier (CFA) tube. Typically, the modulator circuit includes a high power switch tube with the plate electrode thereof serially connected to the cathode of the radio frequency tube, the anode of the radio frequency tube being grounded, and, the cathode of the switch tube being serially coupled to the negative terminal of a positive grounded high voltage power supply. Thus, radio frequency energy fed to the input port of the radio frequency tube is amplified in the radio frequency tube and is coupled to the output port thereof when the high voltage power supply is electrically coupled to such radio frequency tube by the modulator; conversely, the input radio frequency signal is decoupled from the output port of the radio frequency tube when the modulator electrically decouples the high voltage power supply from the radio frequency tube. In this manner, pulsing the modulator results in pulsed amplified radio frequency (RF) energy at the output terminal of the radio frequency tube; such pulsed RF energy having the same pulse width, duty cycle, and pulse repetition frequency as the modulating signal fed to the pulse modulator.
While such pulse modulator has been found useful in some applications, the switching tube used in such circuit usually has a short operating lifetime when compared to the radio frequency tube, and thus, such switch tube is a significant contributor to transmitter maintenance, material and workload. Further, the heater power required with such switch tube consumes significant prime power and contributes to overall transmitter inefficiency since, inter alia, they require high voltage drops because of high plate resistance, require a number of high voltage supplies for biasing. Still further, the switch tubes are very susceptible to damage in a high shock and vibration environment. Thus, overall, the switch tubes have demonstrated a relatively low mean time between failures (MTBF).
One technique suggested to eliminate the use of the switch tube is to use a solid state device, such as a transistor, in its place. The use of a single transistor, however, is not practical for high voltage applications where the transistor will have developed across it the high voltage of the supply when the transistor is in the non-conducting state. One technique suggested to remove this excessive voltage condition across the transistor is to provide a plurality of serially coupled transistors between the high voltage power supply and the load. With such arrangement, however, the drive signals for each of the transistors must be generally biased to a different relatively high voltage potential. To provide such drive signals typically requires the use of a tapped transformer or series of resistors to provide the properly biased control signal for each of the serially coupled transistors thereby reducing the desirability of such an arrangement because of resonances, time delays, and power loss with the tapped transformer or series of resistors.